Modern aircraft are commonly equipped with relatively large numbers of LRUs; that is, modular aircraft components or packages, which contain microelectronic devices and which are designed for rapid, in-field replacement. A non-exhaustive list of aircraft components commonly implemented as LRUs includes: heat exchangers, oil and fuel pumps, cooling fans, various valves (e.g., surge control valves, fuel metering valves, and bleed air valves), starter motors, various sensors (e.g., temperature and pressure transducers), ignition control units, and engine control units. By virtue of their modular nature, avionic LRUs can be quickly replaced or interchanged on an as-needed basis during unplanned maintenance events. An unplanned maintenance event may occur when, for example, an aircraft engine fails to start or responds sluggishly during MES. Maintenance personnel may be dispatched to the aircraft to determine if proper engine operation can be restored by replacing an LRU involved in the MES procedure, such as the starter air valve or the air turbine starter in the case of an air-started engine. If maintenance personnel can determine that a particular LRU is likely at fault, the LRU may be replaced and MES may be reattempted. In many cases, such actions can rapidly restore aircraft operation with minimal downtime to reduce operational costs, improve on-time delivery, and avoid flight cancellation. Avionic LRUs thus serve a vital role in aircraft design and operation.
Avionic LRU failures occur for a variety of reasons. In many cases, avionic LRU failures stem from structural compromise of electrical interconnections between the electrically-active components of the LRU, such as the LRU connector terminals and the microelectronic devices housed within the LRU. Such interconnection failures are often solder-related and may occur due to solder migration or crack formation in the presence of the elevated temperatures, repeated thermal cycling, and high vibratory loads typical of the avionic environment. In certain instances, high temperature solders can be utilized to help mitigate such LRU failure modes, but often contain lead and may provide only a modest increase in the overall temperature tolerances of the LRU. Furthermore, the inclusion of PWBs and other organic components may also place undesirable temperature constrains on LRU operation and may detract from LRU reliability over repetitive and pronounced thermal cycling conditions; e.g., in the case of engine controls, it is not uncommon for an LRU to be subjected to repeated thermal cycling with peak temperatures approaching 175° C. and minimum temperatures approaching −55° C. Due, at least in part, to such temperature limitations, LRU-containing aircraft systems are commonly designed to remotely locate avionic LRUs from local high temperature environments, which adds undesired cost and complexity to the aircraft system.
There thus exists an ongoing industry demand for avionic LRUs capable of operating within the high temperature, high vibratory aircraft environment over prolonged periods of time and across repeated thermal cycles with enhanced reliability. In addition to providing failure-resistant high temperature operation, it would be desirable for such high temperature avionic LRUs to be amenable to manufacture in an efficient manner and at production costs comparable to, if not less than those associated with the manufacture of conventional LRUs. Finally, it would be desirable for such high temperature LRUs to further have relatively compact, lightweight, and structurally-robust constructions; and, in at least some instances, to provide additional flexibility in LRU interconnection schemes and the overall modularity of LRU-containing aircraft systems. Embodiments of such high temperature LRUs and LRU-containing aircraft systems are provided herein, as are methods for manufacturing such high temperature avionic LRUs. Other desirable features and characteristics of embodiments of the present invention will become apparent from the subsequent Detailed Description and the appended Claims, taken in conjunction with the accompanying drawings and the foregoing Background.